Dye-sensitized solar cells are solar cells in which a dye that absorbs sunlight releases electrons upon absorbing light, thereby generating electricity. An article published in 1991 by Michael Gratzel of the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland initiated the research into these solar cells. In terms of the cell mechanism, when light enters the cell, the dye inside the cell enters an excited state, thereby releasing electrons. These electrons are transported through titanium oxide (TiO2) to a transparent electrode, and then flow out of the cell. Meanwhile, the dye that has been converted to cations as a result of the electron release receives electrons that have been supplied from the other electrode via iodine (I) within the electrolyte, and is thereby returned to its original state.
Examples of the types of properties required for the transparent electrode used in these types of solar cells include low resistance, thermal stability and chemical stability, high light transmittance, moisture resistance, and low cost. As the transparent conductive film of an electrode that satisfies these properties, a fluorine-doped tin oxide film (FTO film) is preferred to the more typical tin-doped indium oxide film (ITO film), as it exhibits superior stability under thermal and chemical conditions.
However, because ITO films exhibit excellent transparency and conductivity, they are widely used in liquid crystal display elements and solar cells, and therefore films in which an FTO film is laminated on top of an ITO film have also been developed. One example is reported in Patent Document 1.
Patent Document 1 discloses that an ITO film thickness of 100 nm to 1,000 nm and an FTO film thickness of at least 30 nm to 350 nm are ideal, and also discloses that provided the FTO film is of this thickness, the laminated film exhibits no reduction in conductivity even when heated at a temperature of 250 to 700° C. for one hour. Further, the document also discloses that deposition of the FTO film must be performed continuously following deposition of the ITO film. Hence, it is disclosed that the raw material compound solution that generates the FTO film must be sprayed immediately following formation of the ITO film, while the vitreous film is still at a temperature of approximately 400 to 500° C., so that the FTO film is formed by a spray pyrolysis deposition method (SPD method) before the ITO film begins to deteriorate.
However, this method is not entirely satisfactory, because the overall film is quite thick, meaning problems of cost tend to arise, and the thickness of the FTO film is also quite large, meaning the advantages of the ITO film do not manifest adequately.